In the pulse compression technology described in Non-patent Document 1, a pulse laser beam (hereinafter simply referred to as light pulse) outputted from an optical fiber laser source with a central wavelength of 1.06 μm is amplified by an Yb-doped optical fiber amplifier, the light pulse thus amplified is expanded by an optical fiber, and then the light pulse is compressed by a diffraction grating. By using such light pulse compression technology, femtosecond laser sources, whose pulse width is 100 fs or less at a wavelength of 1.06 μm, can be obtained. The optical fiber used for expanding light pulses in Non-patent Document 1 had a β2 of 0.10 ps2/m and a β3 of −0.00018 ps3/m at the wavelength of 1.06 μm .
Herein βn is the n-th derivative of the propagation constant β with respect to frequency ω. That is, the propagation constant β is given by following formula (1) as a Taylor expansion around the central frequency ω0 of the light pulse, while the n-th derivative βn at the frequency ω0 is given by the following formula (2). The following formula (3) and formula (4) define the relationship between the chromatic dispersion D, the dispersion slope S and the second derivative β2 and the third derivative β3, so that the latter can be mutually converted on the basis of the formulas. In the formulas, c is the speed of light in vacuum, π is the circular constant, and λ is the wavelength of the light.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                                β              =                            ⁢                                                β                  0                                +                                                      ∑                                          n                      =                      1                                        ∞                                    ⁢                                                            1                                              n                        !                                                              ⁢                                                                                            β                          n                                                ⁡                                                  (                                                      ω                            -                                                          ω                              0                                                                                )                                                                    n                                                                                                                                              =                            ⁢                                                β                  0                                +                                                      β                    1                                    ⁡                                      (                                          ω                      -                                              ω                        0                                                              )                                                  +                                                      1                    2                                    ⁢                                                                                    β                        2                                            ⁡                                              (                                                  ω                          -                                                      ω                            0                                                                          )                                                              2                                                  +                                                      1                    6                                    ⁢                                                                                    β                        3                                            ⁡                                              (                                                  ω                          -                                                      ω                            0                                                                          )                                                              3                                                  +                …                                                                        (        1        )                                [                  Formula          ⁢                                          ⁢          2                ]                                                                      β          n                =                                                            ⅆ                n                            ⁢              β                                      ⅆ                              ω                n                                              ⁢                      |                          ω              =                              ω                0                                                                        (        2        )                                [                  Formula          ⁢                                          ⁢          3                ]                                                            D        =                              -                                          2                ⁢                π                ⁢                                                                  ⁢                c                                            λ                2                                              ⁢                      β            2                                              (        3        )                                [                  Formula          ⁢                                          ⁢          4                ]                                                            S        =                                            4              ⁢              π              ⁢                                                          ⁢              c                                      λ              3                                ⁢                      (                                          β                2                            +                                                                    π                    ⁢                                                                                  ⁢                    c                                    λ                                ·                                  β                  3                                                      )                                              (        4        )            
Standard single-mode optical fibers, which are used ordinarily in optical transmission and have a zero-dispersion wavelength in the vicinity of the wavelength of 1.3 μm, exhibit a second derivative β2 of 0.02 ps2/m and a third derivative β3 of 0.00004 ps3/m at the wavelength of 1.06 μm. As compared with such standard single-mode optical fibers, optical fibers used for expanding light pulses, such as the one described in Non-patent Document 1 (hereinafter referred to as optical fiber for light pulse expansion) have a second derivative β2 of the same sign (positive) and a third derivative β3 of the opposite sign (negative). Patent Document 1: Japanese Patent Application Laid-open No. 2002-293563 Non-patent Document 1: M. E. Fermann, et al., Advanced Solid-State Lasers Topical Meeting in Seattle, 2001, Technical Digest TuA3, pp. 355-358